Process for preventing corrosion and the formation of scale in water circulating system

ABSTRACT

Preventing corrosion and scale formation in water re-circulating systems by adding to each cubic meter of the water being re-circulated from about 5 to 100 grams of a compound selected from the group consisting of those of the formula ##EQU1## and the water-soluble salts thereof R being hydrogen, alkyl, alkenyl or alkinyl having up to 4 carbon atoms; phenyl; cycloalkyl having 3 to 6 carbon atoms; benzyl; phenethyl or ##EQU2## wherein R&#39; is hydrogen, alkyl having 1 to 4 carbon atoms or carboxyl, R&#34; is hydrogen or methyl and R&#39;&#34; is carboxyl.

This invention relates to a process for preventing both corrosion andthe formation of scale in water-conducting systems which comprisesadding a substance based on phosphonic carboxylic acid or its salts tothe water.

The main problems arising in hydraulic engineering, e.g. when usinguntreated water for cooling purposes are the occurrence of corrosion andthe formation of scale.

The corrosion of metals such as steel, aluminium, copper and alloys ofthese metals which are commonly used for water circulation tanks ismainly due to the action of oxygen and carbon dioxide dissolved in thewater.

The prevention of corrosion by removing the oxygen, for example by theaddition of hydrazine or sulphide, is impossible for economical andtechnical reasons in open cooling circuits, and corrosion inhibitorssuch as chromates or polyphosphates are therefore added to the coolingwater.

Although chromates are, no doubt, very efficient corrosion inhibitors,their use is often undesirable since it often entails considerabledisadvantages on account of their well known toxic effects.

The formation of scale in a cooling circuit, due mainly to thedeposition of carbonates, sulphates, phosphates and hydroxides ofcalcium and magnesium on the heat transfer surfaces, impedes thetransfer of heat in the heat exchangers and, therefore, reduces theefficiency of the plant.

Furthermore, the deposits of scale are liable to promote corrosion dueto the formation of pockets of aeration.

Apart from the addition of corrosion inhibitors, polyphosphates havepreviously also been added to cooling water in order to preventformation of scale. The action of the various polymeric phosphatesconsists in forming water-soluble complexes with calcium and magnesiumwhich would otherwise harden the water. Also polyphosphates have aso-called threshold effect, that is to say when added in less thanstoichiometric quantities they are capable of preventing the formationof a firm scale deposit by disturbing the growth of the crystal lattice.

Polyphosphates have, however, the serious disadvantage of being unstableat the temperatures at which they are employed and of readilyhydrolysing to the ortho-phosphate which thus interferes with theprotection against corrosion as well as soiling the circulating water bythe inevitable formation of sludge. Another disadvantage of thephosphates is their property of promoting the growth of algae in thecooling circuit and having an undesirable eutrophic action on water.

We have now found a process for preventing corrosion and the formationof scale in water conducting systems, in particular in cooling waterre-circulating systems which is characterised in that one or morephosphonocarboxylic acids or their water-soluble salts and optionallyother corrosion inhibitors are added to the water.

Water-soluble salt means that the salt is soluble in water under theherein referred to application conditions. Such salts are preferablyalkaline, alkaline earth, ammonium or zinc salts.

Phosphonocarboxylic acids are understood in the context of thisinvention to be those phosphonocarboxylic acids which contain more thanone carboxyl group.

Phosphonocarboxylic acids which are suitable for the process accordingto the invention have the following general formula. ##EQU3## wherein Rrepresents a hydrogen atom, an optionally substituted alkyl, alkenyl oralkinyl group, in each case having up to 4 carbon atoms, an aryl,cycloalkyl or aralkyl group or the group ##EQU4## in which R' representshydrogen, an alkyl group having up to 4 carbon atoms or a carboxylgroup;

R" represents hydrogen or methyl; and

R"' represents a carboxyl group.

These phosphonocarboxylic acids are particularly distinguished by theirlow phosphorus content combined with their high lime binding capacitycompared with the conventional polyphosphates. Another importantadvantage is their resistance to hydrolysis and the thermostability ofthese phosphonocarboxylic acids compared with the usual polyphosphates,especially at high temperatures.

The following are specific examples of such phosphonocarboxylic acids:

α-Methyl-phosphonosuccinic acid;

Phosphonosuccinic acid;

1-Phosphonopropane-2,3-dicarboxylic acid; and

2-Phosphono-butane-tricarboxylic acid-(1,2,4).

The preparation of such phosphonocarboxylic acids has been describede.g. in German Offenlegungsschrift No. 2 015 068.

The quantity of phosphonocarboxylic acids or salts used may vary withinwide limits but it is advisable to add about 5 to 100 g/m³ ofphosphonocarboxylic acid based on the water.

The action of the phosphonocarboxylic acids or their derivatives can beconsiderably improved by the addition of zinc salts, e.g. zinc sulphate,zinc phosphate, and/or phosphoric acid or phosphates which surprisinglyhave a distinct synergistic effect. Also any of the additives mentionedin H.O. Held, Kuhlwasser, Vulkan-Verlag, Essen, 1970, page 218 may beused.

The invention will now be explained more fully with the aid of thefollowing specific examples. The phosphonocarboxylic acid mentionedthere (phosphonobutanetricarboxylic acid) is particularly readilyavailable. The accompanying figure illustrates the inhibitory action of2-phosphono-butanetricarboxylic acid-(1,2,4) on carbon steel (St 35) inexperiments carried out with stirrers (room temperature, pH 6.5 to 8.0).The rate of corrosion is entered along the ordinate ##EQU5## and thequantity of phosphonocarboxylic acid used is entered along the abscissa(in ppm).

EXAMPLE 1

This example illustrates the inhibitory effect of2-phosphonobutane-tricarboxylic acid-(1,2,4) on the corrosion of carbonsteel St 35 (the steel used for cooling towers).

Sections of St 35 pipe which had been annealed under conditions ofstress relief and pickled with hydrochloric acid before the experiment,were attached to a plastic stirrer and displaced at the rate of 0.63 m/sin tap water (total hardness 17.5°d, temporary hardness 7.5°d, 110 ppmSO₄ ⁻ ⁻, 185 ppm Cl⁻, total salt content approximately 700 ppm) to whichincreasing quantities of 2-phosphono-butanetricarboxylic acid-(1,2,4)were added. The experiment was carried out at room temperature. The pHof the water fluctuated in the range from 6.8 to 8.4. When theexperiment had been carried out for 4 days, the test samples were againpickled and the total loss was determined. In the attached figure, theweight loss of the pipe sections expressed in g Fe/m² .d is plottedagainst the concentration of 2-phosphonobutanetricarboxylicacid-(1,2,4). The loss of Fe drops very sharply with increasingconcentrations of inhibitor but rises again very slowly atconcentrations above 160 ppm of 2-phosphono-butane-tricarboxylic acid-(1,2,4). The formation of the corresponding iron complex in the presenceof oxygen is very noticeable here (see figure).

EXAMPLE 2

In an experiment carried out as described in Example 1, the loss of Fefrom St 35 steel in tap water which had not been treated with inhibitorwas 15.97 g/m².d. The loss of Fe in the presence of 30 ppm of2-phosphono-butane-tricarboxylic acid-(1,2,4) was 1.07 g/m².d whereas ifin addition 10 ppm of Zn⁺ ⁺ were added in the form of zinc sulphate, therate of corrosion was reduced to 0.32 g/m²,d.

When Zn was added alone in substantially the same quantity, theresulting effect was less marked. The loss of Fe by corrosion measuredin this case was 11.99 g/m² .d.

EXAMPLE 3

An additional synergistic effect is obtained by adding phosphate in theform of phosphoric acid or alkali metal phosphates to the combination of2-phosphono-butanetricarboxylic acid-(1,2,4) and zinc.

Under the experimental conditions described in the previous examples, acombination of 30 ppm of 2-phosphonobutane-tricarboxylic acid-(1,2,4)and 10 ppm of Zn⁺ ⁺ resulted in a loss of Fe by corrosion of 0.32 g/m².d. If in addition 10 ppm of PO₄ ⁻ ⁻ ⁻ were added in the form of H₃ PO₄,the weight loss of the pipe sections was only 0.024 g/m² .d.

Preferred corrosion inhibitory compositions containing 10 to 60% byweight of the indicated phosphonopolycarboxylic acid or salt include 1to 20% by weight of a Zn salt and 5 to 30% by weight of H₃ PO₄, thetotal adding up to 100%. We claim:

1. A process for preventing corrosion and the formation of scale in awater re-circulating system which comprises adding to each cubic meterof the water being re-circulated from about 5 to 100 grams of a compoundselected from the group consisting of those of the formula ##EQU6## andthe water-soluble salts thereof, R being hydrogen, alkyl, alkenyl oralkinyl having up to 4 carbon atoms; phenyl; cycloalkyl having 3 to 6carbon atoms; benzyl; phenethyl or ##EQU7## wherein R' is hydrogen,alkyl having 1 to 4 carbon atoms or carboxyl, R" is hydrogen or methyland R'" is carboxyl.
 2. The process of claim 1 wherein the compound ofsaid formula is 2-phosphono-butane-tricarboxylic acid-(1,2,4) or a watersoluble base addition salt thereof.
 3. The process of claim 1 wherein anadditional corrosion inhibitor is added to said water to bere-circulated.
 4. The process of claim 3 wherein said additionalcorrosion inhibitor introduces phosphate or zinc ions into solution. 5.The process of claim 1 wherein the salt added to said water is the zincsalt of 2-phosphono-butane-tricarboxylic acid-(1,2,4).
 6. The process ofclaim 1 wherein said water re-circulating system is a cooling watercirculating system.